A young hand holds an wrinkled hand. Photograph: Montgomery Martin/Alamy

Care workers are being paid as little as £5 an hour – well below the legal minimum wage – according to a new report from a thinktank, which describes the practice as a "national scandal".

By examining the "work diaries" and payslips of care workers doing home visits to show how their real pay levels do not reflect the hours they work, the Resolution Foundation found that a typical working day can run from 7am to 10pm, with care visits concentrated in morning and evening slots to help clients bathe, dress and eat.

Workers are given time slots in which to complete each visit, which can be as little as 15 minutes, for which they are paid. But the time taken to travel between appointments, sometimes involving journeys of more than 20 miles, are often unpaid – driving their overall pay rate below the legal minimum.

What the foundation calculates is that while headline pay rates for care workers who visit clients at home are set at or above the national minimum wage of £6.19 an hour, in practice, those workers often lose at least £1 an hour because the time spent travelling between appointments is unpaid and the time allocated by the employer for each visit is not enough to provide care. This would mean that over the course of a year, a care worker who spent an average of 35 hours a week at work for 48 weeks would lose out on more than £1,600.

A study in 2011 by King's College London's social care workforce research unit estimated that there were between 150,000 and 220,000 care workers earning below the minimum wage.

There are an estimated 2 million care workers in the UK, 830,000 of whom are "domiciliary" care workers, carrying out home visits. Last year HMRC, which is responsible for enforcing the minimum wage, served notices on 879 employers from every industrial sector claiming that staff had been underpaid.